Apparatus for thermal imaging

ABSTRACT

A device for thermal imaging of target surfaces includes a housing ( 12 ) with an opening ( 14 ) for directing incident infrared rays along an optical path through an optical assembly ( 40 ) optimized to have a spectral band width of 3 to 14 um, onto a UFPA detector ( 48 ) having a spectral transmission window ( 84 ) which has a bandwidth 3 to 14 um sufficient to pass all infrared rays of interest over a broad temperature range. Filter means ( 44 ) including at least two filters ( 78, 80 ), having a band width in the ranges of 3 to 8 um and 8 to 14 um, respectively, allow for the placement of either specialized IR filter in the optical path so as to attenuate and/or pass certain wavelengths of the infrared rays depending on the specific application in a broad range between −40° C. to 2000° C. The device allows for thermal imaging even in daytime applications in sunlight.

FIELD OF THE INVENTION

This invention relates to an apparatus for providing thermal images inunique situations, for example the thermal imaging of the wall surfaceof tubes used in direct-fired, process heaters, and to such an apparatuswhich further allows for temperature measurements associated withconventional, predictive preventive maintenance PPM applicationsinvolved with the industrial process, or otherwise at the facility, asthe user identifies and selects.

BACKGROUND

There are classes of applications in petrochemical and power utilityindustries that involve the thermal imaging and absolute temperaturemeasurement of wall-tube surfaces in direct fired process heaters underproduction conditions. The temperature range in these processes variesfrom 400 to 1200° C. It is widely recognized that operation of furnacetubes above their creep-rupture design temperature, for example inethylene plants, results in diminished lifetimes and increases theprospects for premature failures. Any failure of the tubes results invery expensive repair costs and furnace shut downs.

In addition, temperatures often adversely affect productivity or yieldof desired product. For example a 10° C. temperature difference fromdesired temperature at the coil output of a large ethylene plant canresult in hundreds of thousands of dollars of revenue loss per year.Such a loss is caused by a less than desired conversion of thefeedstock. The same thing applies to coker furnaces in refineries. Whenoperating at a slightly higher temperature than optimum, overcaking andincreased coke formation inside of the tubes results. This causes higheroutside temperature, and consequently reduced through puts. Formation ofcoking inside tubes and identifying the exact location of this formationis most important. Large uncertainties in the tube surface temperaturesare unacceptable if the process is to operate under nearly optimizedconditions with any degree of confidence.

Power utilities are becoming more cost conscious as the result ofderegulation. Much of the same principles detailed above apply toutilities' furnaces. In a coal fired utility furnace, for example,identifying clinker formation inside the boiler tubes is as important asidentifying coke formation in petrochemical cracking furnaces.

The industry's techniques for the furnace tube temperature measurementhave improved through the years and there exist three methods at thepresent time for this measurement.

The first method utilizes a thermocouple with direct physical contact,such as welding to the tubes in select locations. However, thermocoupleinstallations are unreliable for extended operation because of the rapiddrift in their calibration; and, deterioration of the protectivematerials in the furnace atmosphere. In addition the number ofthermocouples installed is limited due to the complexity which resultsin the associated wiring and instrumentation. Normally distances of 100meters and longer are necessary to reach the control room. It is nearlyimpossible to identify the exact location of tube coking by thethermocouple method.

A second method, which is widely used in many plants employs portable,single point radiation thermometers with appropriate optics, spatialresolution and infrared filtering. These instruments have the ability tocorrect for the effects of in-furnace conditions such as emissivity,reflected irradiance and furnace gas emission/absorptions on theindicated radiation thermometer readings (see literature for Mikronmodel M90D and Mikron/Quantum Logic model 1, both manufactured by MikronInfrared, Inc. of Oakland, N.J., (hereinafter “Mikron”, “Assignee”and/or “Applicant”) for more details). In the Mikron/Quantum Logic I anovel method of using a modulated laser permits measuring the emissivityof the tube, allowing more precise temperature measurement of the tube.

These conventional, single point, portable radiation thermometers haveone serious shortcoming, i.e., it is nearly impossible to expect someoneto measure all the tubes across the entire length or height of thefurnace. The number of measurements can easily reach hundreds perfurnace per day. Operator fatigue and boredom will eventually result inthe deterioration of the quality of the reported data. Consequently theprocess engineers choose only select points for measurement and ignorethe rest of them. Thus, the identification of locations where cokeformation takes place, becomes more a matter of chance than a certainty.

The third method presently employed uses a thermal imaging instrumentwith a sufficient field of view to observe a very large portion of theinterior of furnace. FIG. 1 shows a cross section of a typical cokerfurnace in a refinery. The fields of view 1, 3, 5 for different imagerpositions 7, 9, 11 are depicted. A sufficient number of viewports 13, 15are available in order to image a substantial if not all of the interiorof the furnace. These mid-wavelength, infrared (MWIR) instrumentsinclude a suitable infrared filter which allows the imager to “see”through a substantial depth of hot combustion gases 17. A typicalinfrared filter is a narrow pass band filter centered at 3.90 um. Flamecombustion by-products include gases such as H₂O, N₂, CO₂, and NO_(x),and a small residue of ashes and other particles. These hot combustiongases emit a substantial amount of radiation toward the wall tubes 19resulting in heating the tubes. It is known that at 3.90 um there is avoid in the spectrum of hot gases radiation (see FIGS. 2A and 2B) thatmakes the hot gases very transparent. An instrument operating at thisparticular wavelength where the target is absorptive and thus emissive,can provide a very high quality thermal image of the interior of thefurnace even in the presence of hot combustion gases.

In addition by estimating or knowing the emissivity of the tubes andfurnace background temperature for calculation of tube reflectedirradiance, one can get adequate repeatability. The sensitivity of thethermal imagers is quite good such that differences of 1-2° C. can beeasily discerned. However, the matter of establishing absolutetemperature levels on tubes is quite another matter.

Modern thermal imagers have the ability to store the images taken in thefield for further off-line image processing. A number of usefulparameters and in particular temperature profile/time trend analysis canbe readily determined. The trend of wall tube temperature in most casescan effectively be used as an indication of the expected life of thetubes or formation of coke inside of the tubes, either one of whichhaving a substantial effect on the productivity of the process and theover all cost of operation of the plant.

Present State of the Art in Thermal Imaging

The existing thermal imagers designed with an appropriate infrared bandpass filter of 3.9 um for penetration through hot combustion gases relyon photon detectors such as Indium Antimonite (InSb), Mercury CadmiumTelluride (MCT), Platinum Silicide (PtS) or Quantum Well InfraredPhoto-detector (QWIP). A typical detector has an array of 320 H×240 Velements (pixels) to form a thermal image and are very sensitive in thespectral band of 3 to 5 um. The main shortcoming of this class ofdetector is that they have to operate at very low cryogenictemperatures, such as 77 K, which is equivalent to the temperature ofliquid nitrogen.

To achieve cryogenic temperatures for a portable instrument demands avery high-tech cryocooler, which operates on the same principles as ahouse refrigerator, except that helium gas or other very low temperatureliquid gas is used as the medium of compression. In addition to theinitial manufacturing costs, incorporating a cryocooler compressor intoa portable instrument has other shortcomings. Firstly, in order for acryocooler to reach sufficiently low cryogenic temperatures it takesseveral minutes. Second of all, a cryocooler has many moving and sealingparts such as piston, cylinder, gaskets, o-rings and motor. The piston,cylinder, gaskets and o-rings seals must operate under very highpressure, in order to convert gas to liquid. The typical life of acryocooler is about 2000 hours. A normal failure mode is the leaking ofhelium gas through the seals. The replacement or repair of a cryocoolercan exceed 25% of the initial cost of buying the instrument. Besidesbeing costly, repairs normally are associated with long delays dueeither to spare parts' shortages or the limited number of repair peoplewith the necessary level of expertise. Further, the battery life ismostly consumed in keeping the detector cooled. Normally operators mustcarry an external high capacity battery, either strapped over theshoulder, or belted around the waist. This adds to the inconvenience andposes a threat to the safety of the operators, since operators mustimage the interior of these furnaces from narrow catwalks through hotview ports several stories high. Of course, the avoidance of injuriesduring this operation is of paramount importance to plant management.

Further, these photon detectors are effectively limited, again, to thespectral band of 3 to 5 um. As such, they are not useful in detecting“lower” temperatures, for example, in the range of −40 to 200° C., andparticularly outdoors, during the day, in sunlight, which, due to theinfluence of the sun, a powerful source of radiation energy at 3 to 5um, precludes their use. These lower temperatures can occur at otherpoints in petrochemical-related processes and can also be very critical.Monitoring of these conditions is usually accomplished using a long waveinfrared (LWIR) imaging radiometer operating in the 8-14 um spectralbands.

Also, ancillary furnace and other facility functions can be the subjectof a comprehensive predictive and preventive maintenance (PPM) programrequiring a similar low temperature, detection capability.

Thus the present state of the art requires the use of two instruments tocover the broad temperature range of −40 to 2000° C. for two distinctlydifferent applications, such as coker furnaces and PPM activities.

In the last several years a class of un-cooled focal plane array (UFPA)infrared detectors has been introduced to the commercial market fornumerous industrial, scientific, security, public safety, automotive andfire fighting applications. The impetus for the design and developmentof these modern detectors was the need by the military for a light,highly potable night vision device. The main advantage of UFPA detectorsis that they can operate at room temperature. There is no need for acryogenic environment to cool down the detectors.

However all these detectors are optimized to operate for the longerwavelengths of the infrared spectrum, normally beyond 6 um. The detectoris vacuum-sealed for optimum performance by an infrared transmittingwindow covering the sensitive sensing elements (pixels). The infraredtransmission characteristic of this window is from 6 to 14 um or 8 to 14um. The spectral transmission of 8 to 14 um is preferred choice, sincethe effect of atmospheric absorption band is minimized, thus allowinggreater clarity of images at longer distances.

It is a primary object of this invention to provide for a new use forthese UFPA devices by adapting them in a way that permits them to beused as MWIR devices in addition, so that they are able to measure thehigh temperature of target surfaces, for example the tube walls inside afurnace without the interference from combustion flames, as well asfunctioning in low temperature ranges, including detecting temperaturesin broad daylight.

It is a further object of the present invention to provide a portablelightweight instrument which does not require cooling to cryogenictemperatures thereby prolonging instrument battery life therebyproviding additional convenience and safety that is very desirable insuch environments.

A still further object is to adapt existing UFPA thermal imaging devicesso as to accomplish the purposes of the present invention, the deviceincluding built-in firmware and associated off-line software, forexample, MikroSpec™ off-line software, to further enhance the degree ofaccuracy of the measurement, allowing for further temperature/time trendanalysis which can for example prolong the life of the tubes and thusthe productivity of plant operation, or provide other benefits when usedwith different processes.

SUMMARY OF THE INVENTION

Towards the accomplishment of these and other objectives and advantageswhich will become apparent from a reading of this specification takentogether with the accompanying drawings there is provided a device forthermal imaging of target surface(s) having different temperatureswithin a range of temperatures of interest between a high and lowtemperature of −40° C. to 2000° C. The thermal imaging takes placethrough intervening media having a known transmission wavelength. Thetarget surface(s) have a known absorptive wavelength.

The device comprises a housing including an opening for admittinginfrared rays including those emanating from the target surface(s). Therays are directed along an optical path within the housing. The opticalpath has an optical axis.

An optical assembly is positioned within the housing and in the opticalpath. The optical assembly has an input and an output. The infrared raysare directed towards and into the input, through and out of the outputof the optical assembly.

Means for optimizing the spectral band width of the optical assembly to3 um to 14 um are provided. In the preferred embodiment, the opticalassembly includes an objective lens, a negative lens, and focusing lensmeans. In the preferred embodiment, each of the lenses is made ofgermanium. In the preferred embodiment, each lens has an anti-reflectioncoating with a spectral band width of 3 um to 14 um.

There is also present an un-cooled focal plane array, infrared raydetector (UFPA detector) which includes a detecting surface. The UFPAdetector is positioned in the housing and in the optical path so as toallow the impingement of the infrared rays passing out of the opticalassembly onto the detecting surface.

Means for optimizing the spectral band width of the UFPA detector to 3um to 14 um are employed. In the preferred embodiment, this includes aspectral transmission window positioned in the optical path between theoutput of the optical assembly and the UFPA detecting surface, with thespectral transmission window having a spectral band width of 3 um to 14um. Typically the transmission window is made part of the UFPA detector.

The UFPA detector provides an electrical output proportional to theenergy of the infrared rays impinging onto the detecting surface;

Filter means including a first and second infrared band pass filter areprovided. The first infrared band pass filter has a spectral band widthof 8 to 14 um. The second infrared band pass filter has a respectivespectral band width falling within the band of 3 to 8 um. Each of theband pass filters is removably interposed in the optical path upondirection of an operator for filtering the infrared rays entering thehousing so as to attenuate certain infrared rays and to pass otherinfrared rays of particular, respective predetermined wavelengthsassociated with the range of temperatures of interest, the transmissionwavelength of the intervening media and the absorptive wavelength of thetarget surface(s).

Electronic means are provided which are adapted to convert theelectrical output of the UFPA detector into at least one interpretableoutput whereby an operator is presented with information sufficient todetermine the temperature(s) of the target surface(s) within anacceptable degree of accuracy.

The device allows for the thermal imaging of PPM type applications tooccur in sunlight when said first infrared band pass filter isinterposed in the optical path.

In one application of the device, the spectral band width of the secondband pass filter is 3.8 to 4.0 um.

In another application of the device, the spectral band width of thesecond band pass filter is 4.8 to 5.2 um.

In still another application of the device the spectral band width ofthe second band pass filter is 6.7 to 6.9 um.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages will become apparent from the followingdescription when read alone and/or taken together with the accompanyingdrawings which include:

FIG. 1 depicting in a plan, functional view, a typical coker furnace ina refinery;

FIG. 2A depicting in graph form, the spectral emissivity of combustiongases versus wavelength in um and wave number in cm⁻¹;

FIG. 2 B depicting in graph form, the typical transmission spectra ofcombustion flame in percentage versus wavelength in μ;

FIGS. 3, 3A and 3B showing different perspective views of a modifiedmodel 7200 V thermal imager of the assignee and applicant with allnecessary design changes to implement the present invention;

FIG. 4 depicts in functional schematic form various elements of thethermal imager of the present invention;

FIG. 5 depicting in a side elevational, partially sectional, functionalview of various elements which comprise the present invention;

FIG. 6 depicting a two position, IR filter assembly of the presentinvention;

FIG. 7A depicting in front elevational view the UFPA detector used inthe present invention;

FIG. 7B depicting in side elevational view the UFPA detector used in thepresent invention;

FIG. 8 depicting in a partial, plan functional view, the variouscomponents comprising the total incoming radiation to a thermal imagerpositioned at a viewport;

FIG. 9A showing the thermal image of the interior of a coker furnace ina refinery, as viewed through the viewfinder of the instrument of thepresent invention;

FIG. 9B showing the thermal image of the burner bank on the floor of acoker furnace as viewed through the viewfinder of the instrument of thepresent invention;

FIG. 9C showing a visual image of the same burner bank shown in FIG. 9B,which depicts the hot combustion gases (flame) highlighting theeffectiveness of the apparatus of the present invention;

FIG. 10 showing an image of a typical house hot water heaterillustrating the low temperature applications of the present invention,again with the clarity of the thermal image attesting to the superiordesign of the present invention and its adaptability for dual purposes;

FIGS. 11A and 11B depicting in Table form the determination of thetemperature, in 10° C. increments, of a blackbody using Planck's law fordifferent incident radiant energy at wavelengths of 3.8 um to 4.0 um;and,

FIG. 12 depicting in Table form the determination of the temperature, in1° C. increments between 900 and 1000° C., of a blackbody using Planck'slaw for different incident radiant energy at wavelengths of 3.8 um to4.0 um.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings wherein the same reference numbers areused in various figures to identify the same item, in FIGS. 3, 3A and3B, a version of the Assignee's (Applicant's) Model 7200 V, thermalimager modified to implement the present invention is depicted. Theimproved device for thermal imaging of target surfaces includes ahousing 12 having an opening 14 through which the infrared raysemanating from the target surface(s) are received. The improved deviceis an extremely light weight high-performance, IR camera which isdesigned for comfortable, one-handed point-and-shot operation. It usesan intuitive key pad located on the top of the imager which includescursor controls 16, focus controls 18, menu selection key 20 and modeselection key 22. The menu selection key 20 allows the operator toidentify the temperature range to be viewed.

Battery power is supplied upon activation of power switch 24 (FIG. 3B).Memory card slot means 26 is provided to enable the storage of imagesand data to PCMCIA cards for subsequent review. Images can also beviewed in real time via the video outputs 28 and/or 30 (which is aRS-232C S-video output) and/or through an optional built-in IEEE 1394FireWire® interface, 32. Since the camera is battery operated provisionof course is made for use of an adapter at plug 34 for continuous ACoperation.

The MicroScan 7200V comes standard with extensive on board imageprocessing software. It also can be remotely controlled from a PC usingoptional software also available through the Assignee, which providesadditional analysis and reporting capabilities. Such software ismarketed by the present Assignee under its trademark MikroSpec. TheMikroSpec™ real-time thermal data acquisition and analysis software is awindows-based software program that offers high-speed; real-time dataacquisition and image analysis capabilities. By using one or moreinfrared cameras connected to the software, processes can be measuredaccurately to insure production quality. The software allows the user toview thermal images in real-time as well as those that have beencaptured and stored to the computer's hard-disc drive. The softwareallows the creation of numerous regions of interests in various shapesso that details can be retrieved as to the temperature range within theregions of interest.

Referring now to FIG. 4, the thermal imaging device of the presentinvention is depicted functionally. Infrared radiation received atopening 14 comprises radiation emanating from target 36, radiation fromother sources in the vicinity of the target and radiation reflected fromthe target due to other sources in its vicinity. The infrared radiationarrives at the opening 14 of the imaging device where it is directedalong an optical path within the imaging device having an optical access38. Positioned in the optical path and centered about the optical accessis an infrared, optical assembly 40. The received IR radiation isdirected into, through and out of the optical assembly which includes afocus control means 42 which, as noted hereafter, can be either manuallyor motor driven. As functionally depicted, the rays emanating from theoptical assembly 40 are directed through a filter arrangement having atleast two positions so as to interpose filters of different band widthsconsistent with the purposes of the invention and a respectiveapplication. The stepper motor 46 enables the operator to position thefilter 44 at the different positions as he needs to, in response to thetemperature range choice effected by key 20 and corresponding selection.

The infrared optics assembly 40, as noted collects the infrared energyfrom the target, that is the energy within the field of view of theinstrument and focuses that energy onto an un-cooled focal plane array,infrared ray detector (UFPA detector) 48. The UFPA detector is a typicalapplication consists of 320 H×240 V elements which are sensitive toinfrared energy. The UFPA detector 48 provides an electrical outputwhich is proportional to the energy of the infrared rays impinging onits detecting surface. This output is supplied to a pre-amp 50 foramplification of the minute changes sensed by the UFPA detector elementsin response to the impinging rays.

Parallel with the infrared optics, is a visible optic assembly 52. Thiscollects the visible portion of the electromagnetic spectrum originatingfrom the target so as to create a visible image of the target which isrecognizable by the user. The output of the visible optic assembly 52 isfed through a digital converting module 54 which in turn is fed to theCPU module 56.

The CPU unit arranges, manages, receives or sends all necessaryinstruction to perform the various tasks required, for example, thechange for different temperature ranges initiated by menu selection key20. Some of the interactions with the CPU 56 include amicrophone/speaker attachment 58 to record an operator's voice memo forplayback at a latter time; the interface with the key pad keys, forexample the menu select key 20; and a battery check feature 62 formonitoring the remaining capacity of the battery source to provide anearly warning to the operator. In addition the CPU module 56 interactswith a signal processing component 64 which receives the signal from thepre-amp 50.

The signal processing unit 64 contains a necessary algorithm(s) and/orlook up tables (see hereinafter with respect to Equations 1, 2, 3 and 4and FIGS. 11A, 11B and 12) for conversion of the incoming energydeterminations to actual temperature equivalents.

The output 66 of the signal processing unit is supplied to the IF modulewhich converts the output of the signal processor to different types ofrecognizable outputs including, for example, a thermal image through theviewfinder 70 (see FIGS. 9A, 9B, and 10); recognizable digital outputsfor communication to a PC through the GP-IB/FireWire® 32; an analogvideo signal which can be fed directly to a video monitor; and/or RS232C30 which is reserved for communication with the internal signalprocessing unit, 64 and the CPU unit, 56 for firmware up-dating,modifications and calibration. Further the IF module can provide anoutput 72 for an optional color video display (not shown) attached tothe imager. Still further IF module 68 can provide, through the memorycard slot 26, a capability of inserting a PCMCIA which allows thedetected images to be recorded for later viewing and analysis.

In order to achieve the objective of this invention, the optical designplays an important role. In the preferred embodiment, the quality of theimage has to be maintained over the very wide spectral band of 3 to 14um, instead of a conventional optical assembly design used in presentthermal imagers that optimizes either over the spectral band of 8 to 14um or 3 to 5 um. The broader band width includes optics for thedesirable spectral band centered at 3.9 um for high temperaturemeasurement that can see through a substantial depth of hot combustiongases encountered in furnaces, and the 8 to 14 um range that allows thethermal imaging of low temperature objects in associated processes andpredictive and preventive maintenance applications, even, mostsignificantly, during daytime and outdoors when sun is present.

Referring to FIG. 5, the preferred embodiment of the present inventionincludes the optical assembly 40 that has a very wide-band, spectrallyflat, anti-reflection coating between 3 to 14 um. The optical assemblycomprises 4 lenses.

Lens 74 is an objective lens made from germanium material and opticallycoated for high transmission in the spectrum band range of 3 to 14 um.The ray bundle from the target located any distance from 12″ (30 cm) to200FT (60 m), strikes lens 74 and after some diffraction will beprojected into lens 76.

Lens 76 is a negative lens made from germanium and optically coated fora spectral band of 3 to 14 um. The bundle of rays striking the lens 76will emerge from lens 76 more parallel to the optical axis 38. Thisfeature allows the placement of the infrared filter assembly 44 behindthe lens 76 with a minimum of shift of the critical narrow bandassociated with the center wavelength of the infrared filter to beinterposed in the optical path as discussed hereinafter.

The IR filter assembly mount 44 has in this embodiment two positions,either of which is automatically selected by the associatedmicroprocessor CPU 56 inside the thermal imager in response to menu key20 and the temperature range selected. Referring to FIG. 6, at position78, an IR filter with a spectral band width of 8 to 14 um, for lowtemperature thermal imaging, is inserted in the optical path. Atposition 80, a second infrared filter having a narrow pass bandcentered, in the preferred embodiment, at approximately 3.9 um andhaving a band width of 0.2 um, for high temperature thermal imaging, isinserted in the optical path.

Focusing lenses 82 and 84 combine to allow the bundle of rays emergingfrom the selected IR filter to be focused onto sensitive elements of theun-cooled focal plane array (UFPA) detector 48. The precise focusing isachieved by moving the combination of lenses 82 and 84 toward or awayfrom lens 76. The focusing, for example, can be achieved in the Mikronthermal imager of 7200V or 7515 manually or automatically throughactivation of focus keys 18.

A protective ring 86 houses a window to protect the objective lens 74.This serves to protect the thermal imager in very harsh environments, asfor example, the blowing heat and particles which may be experienced atthe view port of large utility furnaces. The ring can be unscrewed toallow other accessories such as a telephoto or wide angle lens assemblyto be attached to the front of the imager.

At position 78 of the filter assembly 44 shown in FIG. 6, an infraredfilter with a spectral band width of 8 to 14 um allows for lowtemperature thermal imaging in the range typically between −40 to 500°C. This measurement is minimally affected due to absorption by theatmosphere, and allows for long distance thermal imaging and isunaffected by the presence of sun in outdoor applications.

In position 80, the present invention introduces a very narrow bandinfrared filter centered at a wavelength which depends on the presenceof any intervening media and/or the absorptive wavelength range of thetargeted surface. As can be seen from the graphs in FIGS. 2A and 2B, aninfrared filter centered about the 3.9 um wavelength, superimposed onspectral emissivity of hot combustion (flue) gases, avoids theabsorption band of hot combustion gases, a by-product of fossil fuelburning. That is, the combustion gases are transparent at thiswavelength. The band width centered at the 3.9 um wavelength is 0.2 um.Consequently, thermal images of wall tubes inside of petrochemical andutility furnaces can be provided. An accurate temperature profile ofthese wall tubes can be obtained with the application of a properalgorithm (Equations 1 and 2, see hereinafter). A typical temperaturerange for the thermal imager with this filter in place, is 400 to 2000°C.

A stepper motor 46 under control of the microprocessor (CPU) 56 inelectronics 92, will position the infrared filters 78 or 80 in front ofthe detector 48 as dictated by the operator via the select (menu) 20 asshown in FIG. 3.

As noted above, the detector 48 is an un-cooled focal plane array (UFPA)having an active pixel array of 320×240. It can be purchased from DRS,Inc. of Morristown, N.J.—their model number U3000AR. It is based on Vox(Vanadium oxide) microbolometer technology or amorphous silicon (a-Si)technology. These detectors are packaged in a rugged, miniaturizedassembly that incorporates a spectral transmission infrared window 84 asshown in FIG. 7B.

However, since most of desired usages for these devices involved nightvision situations, with a typical background ambient temperature of 300K, or other low temperature applications, the detector was optimized towork in the spectral band of 8 to 14 um. In such applications, the peakinfrared radiation takes place at about 10 um. This is exactly in themiddle of detector sensitivity and detector window spectraltransmission. Since the spectral transmission band of 8 to 14 has anextra benefit of being substantially transparent to atmosphericabsorptions, this makes this spectral band one of the most used ininfrared thermometry and thermal imaging.

The present invention, however, uses a window 84 that is spectrallycoated for the very broad band of 3 to 14 um instead of the conventionalwindow in these detectors with a spectral band of 8 to 14 um. As such,the detector can be used for the dual purposes envisioned by theinvention: 8 to 14 um for low temperature thermal imaging; and 3 to 8 umfor specialized high temperature thermal imaging, all within the sameunit. The interior of the detector is vacuum-sealed for maximumsensitivity of sensing elements in the detector sensitive area 86 (FIG.7A).

Returning to FIG. 5, the shutter 88 is a mechanical flag that operateseither by a command from the operator, or automatically, to periodicallyshield the incoming infrared radiation from the target. This time periodmay last from a fraction of a second to a few seconds. Since thetemperature of the shutter is uniform by the virtue of the design, andalso is known due to the placement of a suitable temperature sensor, notshown, this becomes a way to do a quick test of the integrity of thedetector. Also, the non-uniformity of each individual pixel of thedetector is tested and all off-sets associated with drifts can beeliminated. During the time the shutter is closed, the instrument, ofcourse, is “blind” and not taking any images.

The motor 90 actuates the shutter 88 to block momentarily incomingradiation from the target.

The housing 12 is the same as used with Mikron's standard thermal imagermodels, # 7102, 7200V and 7515. It is a precision die cast, made fromaluminum by an injected molding process.

The viewfinder 70 is the same as used on Mikron's models, # 7102, 7200Vand 7515 and is used for seeing thermal images in indoor or outdoorenvironments, such as FIGS. 9A and 9B (both with filter 80 in place) and10 (with filter 78 placed in the optical path). FIG. 9C is an imagetaken with visible spectrum of a built-in CCD camera inside the thermalimager.

The electronics 92 is substantially the same as that used in Mikron'sthermal imager models, # 7102, 7200V and 7515. However, some changes inthe firmware allows for the selection of the different temperatureranges required by the invention, positioning of infrared filters, andincludes different menu selections. The electronics includes a newalgorithm for high temperature range with the ability to cancel theinfluence of background radiation, as discussed hereinafter. The battery94 is a high capacity lithium ion rechargeable battery, and is also thesame type as used in thermal imager models # 7102, 7200V, and 7515.

A thermal imager for the purposes of this embodiment is a modifiedversion of Mikron's model # 7200V. It is a radiometer calibrated toindicate correctly the temperature of a blackbody source. The procedureto obtain the temperature of the blackbody, from the spectral radianceW_(λ,γ) (watts per um²) reaching one pixel of the thermal imager,involves only the use of Planck's law. It is stated as follows:${{W_{\lambda\quad T}\left( {\lambda_{eff},T} \right)} = {C_{1}{\lambda^{- 5}\left( {e^{\frac{- C_{2}}{\lambda_{eff}T}} - 1} \right)}}},$where λ_(eff) is the effective wavelength of the thermal imager, i.e.,3.90 um, C₁ and C₂ are constants with values of 3.741×10⁻⁴ watts um² and14388 um degree respectively; and, T is the temperature of the blackbodyexpressed in K. For this rare situation, i.e. where the measurement isdone against a blackbody source, the obtaining of temperature fromspectral radiance is directly made and no corrections are necessary.

In a typical application for purposes of this invention, however, wherethe thermal imager is viewing a large area of a furnace interior withinits field of view (see FIG. 1) an image of incoming spectral radianceW(λ,T) impinging on a single pixel is created by the UFPA detector 48,which is comprised of several radiation components. Determining theactual tube wall temperature from an indicated apparent temperature isdifficult.

Consider the situation depicted in FIG. 8. FIG. 8 shows schematically,incoming radiation to the thermal imager from different sourcesincluding the unwanted radiation from the surrounding backgroundincluding radiation from hot combustion gases.

The tube surroundings are comprised of a refractory wall 96, with atemperature of T_(w); opposite side tube banks 98 with a temperature ofT_(b1); and hot combustion gases 100 (shown by several dots in FIG. 8)with a temperature of T_(g). With IF filter 80 in place in the imager'soptical path, irradiance originating from the surroundings incident on afurnace tube is reflected into the instantaneous field of view (IFOV) ofthe imager or a single element (pixel) of the detector 48 at a narrowpass spectral band of 3.90 um. Therefore the total spectral radiancesensed by one pixel of the UFPA detector 48, which we will define asW_(a)(λ,T_(a)) 101 (see FIG. 8), is the sum of the radiance due toemission from the various sources and the reflection from the tube (seeequation 2) and is expressed by the following equation #1,W _(a)(T _(a))=t _(g) {e _(t) W _(t)(T _(t))+W _(r)(T _(bg))}+e _(g) W_(g)(T _(g)),where: W_(a)(T_(a)) is the total energy received within the IFOV of asingle pixel of the imager detector 48 producing an apparent temperatureof T_(a) for tube 102; W_(t)(T_(t)) is the equivalent radiated energyfrom a tube at a tube temperature of T_(t), and having an emissivity of1.0; W_(r)(T_(bg)) is the total reflected energy received due to thebackground influence of the surrounding tube bank, 98, refractory walls96 and hot combustion gases 100; W_(g)(T_(g)) is the total radianceenergy from the hot combustion gases at gas temperature of T_(g); t_(g)is the transmission coefficient of the hot combustion gases; e, is theemissivity of the wall tubes; e_(g) is the emissivity of the hotcombustion gases, i.e., (1−t_(g)); and e_(w) is the emissivity of therefractory wall.

The total spectral reflected energy from a tube is expressed by thefollowing equation 2 as follows:W _(r)(T _(bg))=(1−e _(t))└F_(t,bt) e _(t) W _(b1)(T _(b1))+G _(w,t) e_(w) W _(w)(T _(w))+K _(X,t) e _(g) W _(g)(T _(g))┘,

-   -   where W_(b1)(T _(b1)) and W_(w)(T_(w)) are energy levels emitted        by the background tube banks 98 and refractory walls 96 and        W_(g)(T_(g)) is an energy level, at an effective length, of the        hot combustion gases, all at their respective temperatures; and        e_(w) is the emissivity of the refractory wall.

F_(t,bt), G_(w,t) and K_(g,t) are what could be called view factorcoefficients. These attempt to provide a weighing of each contributingelement of the surrounding background. They vary between 0.0 to 1.0depending principally on the geometry of the furnace and view portlocation with respect to the exact location of the measurement area ofIFOV of the detector.

Equations 1 and 2 are considered generalized measurement equations,since the parameters of the equations are different from pixel to pixel.An intimate knowledge of geometry of the furnace, backgroundtemperatures and view factors, from spot to spot, fuel transmission andemissivity for different fuel types, and dependency of the tubeemissivity with angle should be available, in order to obtain the finaltube, true temperature.

By way of illustration consider the following example with the followinginitial assumptions:

-   true temperature of tube, T_(t), is 1173 K or 900° C. (104 in FIG.    11B);-   background refractory wall temperature, T_(w), is 1373 K or 1100° C.    (106 in FIG. 11B);-   combustion gases temperature, T_(g), is 1973 K or 1700° C. (108 in    FIG. 11B);-   emissivity of tube, e_(w) is 0.88;-   emissivity of refractory wall, e_(g) is 0.90;-   emissivity coefficient of combustion gases, e_(g), is 0.004/(meter    of gas depth) at 3.90 um (see FIG. 2A); and,-   the optical transmission coefficient of combustion gases, t_(g),    i.e., (1−e_(g)), is 0.996/(1.0 meter of gas depth) and for 5.0    meters of gas depth is (1.0-5.0×0.004)=0.98, at 3.90 um; and,-   the view factor coefficients F_(b,t), G_(w,t) and K_(g,t) are    respectively assigned the values 0.90, 0.10 and 1.0.-   Values for W_(bt)(T_(bt)), W_(w)(T_(w)) and W_(g)(T_(g)) are    determined from Table 1 (FIGS. 11A, 11B,) for the associated,    assumed temperatures (104, 106, 108). Table 1 is a conversion table    reflecting the relationship between radiance energy and temperature.    It is generated by integrating Planck's Law over the interval of 3.8    um to 4.0 um, which corresponds to the spectral bandwidth of the    infrared filter 80. It is further assumed that the central pixel of    the thermal imager has a distance, or gas depth, of 5.0 meters from    the tube.

Inserting these assumed numbers in equation 2 and solving forW_(r)(T_(bg)),the total spectral reflected energy from tube 102 in FIG.8 is, or,W _(r)(T_(bg))=(1−0.88[0.90×0.88×0.37353+0.10×0.90×0.60653+1.0×(0.004)×(5.0m)×1.5134]W _(t)(T _(bg))=0.04568.

The addition of this reflected energy would result in an apparent,higher reading than the actual temperature.

Now solving equation 1, the total spectral radiance received by theinstrument becomes,W _(a)(T _(a))=0.98{0.88×0.37353+0.04568}+0.004×5.0×1.5134=0.39717

Referring to Table 2 (FIG. 12), the apparent temperature, T_(a), 110would be approximately 923° C., or about 23° C. above actual temperaturewith an instrument emissivity setting at 1.0; and the influence of thebackground is not cancelled. At emissivity setting of 0.88 for the tubethe temperature would appear to be 971° C., about 71° C. above actualtemperature.

However, if you subtract the amount of reflected energy of the tube,W_(t)(T_(bg))=0.04568 radiated energy of the hot combustion gases andadjust for the transmission through hot combustion gases from 0.98 to1.0 the total energy that would be used for calculation instead wouldbe, $\begin{matrix}{{W_{a}T_{a}} = {{1.0\left\{ {{0.88 \times 0.37353} + 0.04568 - 0.04568} \right\}} +}} \\{{0.004 \times 5.0 \times 1.5134} - {(0.004) \times \left( {5.0\quad m} \right) \times 1.5134}} \\{= 0.3287}\end{matrix}$

At an emissivity setting of 0.88 for the wall tube 102, the total energyused for the calculation of the actual tube temperature is,W _(t)(T _(t))=W _(a)(T _(a))+e _(t).Using table I, FIGS. 11A, 11B and 11C,W _(t)(T _(bg))=0.32871+0.88=0.37353.This corresponds to 900° C., 104, using table 1, indicating notemperature error.

CONCLUSION

As shown in the above illustrations, with a sophisticated algorithm thatincludes all components of radiation from the interior of the furnacereaching the thermal imager, one by one, each of the unwanted componentsof the incoming energy can be accounted for and subtracted by the use ofa proper algorithm, in order to achieve absolute temperature readings.In actual practice the user can ignore some of the less importantcomponents of unwanted incoming energy and still be able to get accuratereadings.

In some less critical process applications, the user would be satisfiedwith monitoring the apparent temperature vs. time to insure that no hotspot is developing so that they may not be overly concerned with walltube, absolute temperature readings. In other situations the user mayprefer to use a weighted average of unwanted incoming energy instead ofindividual components.

The above equations 1 and 2 are a generalized measurement equationsready to address all circumstances.

A simplified version of above relationship can be expressed if we assumethe coefficient view factors are constant and do not depend on thegeometry of the furnace design or location of the view port used by thethermal imager. For example, F_(t,bt)=0.90, G_(w,t)=0.10 and K_(g,t)=0.0for all furnace areas regardless of the position of the thermal imagerwith respect to the measurement areas. This is the condition where apercentage of the background refractory walls seen by the thermal imageris very small. Consequently, most of the reflected energy is frombackground tube banks that are nearly the same temperature as the bankof tubes within the FOV of the instrument. In addition for a veryeffective infrared filter the reflected energy of the view factorK_(g,t) of hot combustion gases can also be ignored and thus is assumedto be equal to 0.0.

Implicit in this analysis is an assumption that the surroundingbackground can be approximated as two different blackbodies at twodifferent temperatures, and that the surfaces are diffuse and followLambert's law. One blackbody represents wall tubes of the opposite sideand the other blackbody represents the opposite refractory walls. Theseare reasonable assumptions for analysis and actual practice, whichattempts to demonstrate the magnitude of the effect of various furnaceconditions on the apparent indicated temperature, T_(a). With theseassumptions, the equations 1 and 2 can be rewritten as,W _(a)(T _(a))=t _(g) {e _(t) W _(t)(T ₁)+W _(r)(T _(bg))}+e _(g) W_(g)(T _(g)),  Equation 3and,W _(t)(T _(bg))=(1−e _(t))[FW _(bt)(T _(bt))+GW _(w)(T _(w))],  Equation4.

The equations 3 and 4 can be considered as simplified measurementequations, which could be programmed into the instrument's firmware. Theoperator can easily input the parameters such as tube emissivity, viewfactors for the background tube bank and refractory wall, and hotcombustion gases' emissivity depending on the fuel type, and averagedistance measurements.

Solving the two equations, 3 and 4, to obtain the apparent temperature,T_(a), using the assumed numbers from the previous illustration,W _(t)(T _(bg))=(1−0.88)[0.90×0.37353+0.10×0.73876]=0.04437W _(a)(T _(a))=0.98{0.88×0.37353+0.04437+0.01513}=0.38821

Referring to Table 2, the apparent temperature using the simplifiedequations 3 and 4, is T_(a)≈914° C., about 14° C. above the actualtemperature of tube when the emissitivity setting is placed at 1.0 andcancellation of background irradiance is made.

The present invention provides a new thermal imager design having a UFPAdetector which includes a new, spectral transmission window which allowsthe incoming radiation from a given target reaching the sensitiveelements (pixels) of the detector to include the radiation spectrum from3 to 14 um. The invention allows for the placement of a narrow pass bandinfrared filter centered at a particular wavelength in the rangegenerally between 3 to 8 um, in front of the detector window only whenthe requirement for the thermal imager is for high temperature thermalimaging and temperature profiling through an absorptive media such ascombustion gases. The transparency of this media will only take place ata narrow band wavelength centered at 3.9 um. A specially designedmechanism adaptable for use in the Mikron thermal imager model 7200Vallows for automatic placement of this infrared filter in or out of theoptical path by the operator. The design of the optical lens assemblyand its infrared coating is optimized, in a standard thermal imager, forexample, the Mikron 7200V, a 4 element germanium lens assembly isoptimized for acceptance of the much wider spectrum of radiation.Instead of the traditional 8 to 14 um, it is optimized for the rangefrom 3 to 14 um.

By incorporating these novel design changes, a standard thermal imager,for example, Mikron's model 7200V series, is enhanced so as to produce anew class of infrared thermal imagers with the following featuresheretofore unavailable to industrial users in a single device.

-   -   1. A thermal imager using an un-cooled focal plane array (UFPA)        detector, which is able to see through an absorptive media such        as hot combustion gases by using an appropriately centered,        infrared narrow pass band filter.    -   2. In other ranges of the instrument which cover the temperature        range of −40 to 500° C., an 8 to 14 um filter is interposed in        the optical path, allowing the lower temperature measurements        associated with conventional, predictive preventive maintenance        (PPM).    -   3. For outdoors PPM applications during day time, when the sun        is present, the long wave pass band of 8 to 14 um blocks the        reflected unwanted radiation of sun light ensuring accurate        thermal images.

Firmware inside the instrument allows for the collection of imageseither with a flash card or directly by a PC. Mikron custom off-lineMikroSpec™ software, available from Mikron for use with its thermalimagers, may be used for a comprehensive image manipulation, trendanalysis and maintenance scheduling and report generation, formanagement review and simplifies this difficult measurement. This novelmethod provides higher absolute accuracy of tube temperature within thefield of view of the instrument, pixel by pixel. The simplified fashionof assuming a hot uniform background temperature and cancelling itseffect, as presently done, may not create the desired accuracy. Thefirmware inside the instrument combined with MikroSpec™, off-linesoftware has the ability to predict and cancel the effects of unwantedincoming radiance toward the thermal imager, pixel by pixel, in order toinfer the true temperature of tubes at every point based on apparenttemperature measurements. The novel method of the present inventionrelies on a more comprehensive understanding of the background radiationinfluence and provides for its cancellation.

Through innovative and novel design changes to a standard thermal imagersuch as Mikron's Model 7200V a new class of thermal imagers can bedesigned that has the ability to perform dual functions This new modelhas a temperature span of −40 to 2000° C. It satisfies the existingdominant market requirements for PPM applications with a temperaturespan of −40 to 500° C., including during the day with sunlight present.In addition, it allows industrial users with large furnaces such aspetrochemical companies and utility power plants, to thermally profiletheir furnaces conveniently and safely at higher temperatures up to2,000° C.

Further, the innovative design allows the use of a standard thermalimager such as Mikron's Model 7200V for other purposes, for example,monitoring the seal temperature between the glass envelope and metal endcap during the fabrication of fluorescent light bulbs. In thisapplication since the process is accomplished in the presence of flame,the use of a filter having a band width of 3.8 to 4.0 um allows theinstrument to see through the flame to detect the temperature at theseal-point, to assure its sufficiency.

So, too in the manufacture of plastic bags of certain plastics, it isimportant to detect the temperature of the process as the bags are beingformed so as to assure the integrity of the bag. Here it is found that asecond filter having a band width of 6.7 um to 6.9 um will allow thethermal imager to accurately read the surface temperature of thetargeted plastic since certain plastics are only absorptive in thisrange, once again assuring the consistency in temperature necessary toproduce a quality product.

A still further application calls for the use of a second filter havinga band width of 4.8 to 5.2 um. Here the utility of the modified imageris directed to the fabrication of tempered glass, such as used in carwindshields. This filter allows for the monitoring of the temperatureacross the glass product at its absorptive wavelength assuring thenecessary uniformity required to achieve a satisfactory product.

In all cases, the use of a second filter allows for an expansion of thestandard imager's capabilities making it an attractive, cost-effectivealternative to present approaches.

Other variations on the embodiments described herein will be apparent.Of course, the breadth of the present invention is not to be construedas limited to that disclosed heretofore but rather dictated by the scopeof the claims which follow.

1. A device (10) for thermal imaging of target surface(s) havingdifferent temperatures within a range of temperatures of interestbetween a high and low temperature of −40° C. to 2000° C., the thermalimaging taking place through intervening media having a knowntransmission wavelength, the target surface(s) having a known absorptivewavelength, comprising: (a) a housing (12) including an opening (14) foradmitting infrared rays including those emanating from said targetsurface(s), said rays directed along an optical path within saidhousing, said optical path having an optical axis (38); (b) an opticalassembly (40) positioned within said housing and in said optical path,said optical assembly having an input and an output, said infrared raysdirected towards and into said input, through and out of said output ofsaid optical assembly; (c) means for optimizing the spectral band widthof said optical assembly to 3 um to 14 um; (d) an un-cooled focal planearray, infrared ray detector (UFPA detector) (48) including a detectingsurface (86), said UFPA detector positioned in said housing and in saidoptical path so as to allow the impingement of the infrared rays passingout of said optical assembly onto said detecting surface; (e) means (84)for optimizing the spectral band width of said UFPA detector to 3 um to14 um; said UFPA detector providing an electrical output proportional tothe energy of the infrared rays impinging onto said detecting surface;(f) filter means (44) including a first (78) and second (80) infraredband pass filter, said first infrared band pass filter having a spectralband width of 8 to 14 um, said second infrared band pass filter having arespective spectral band width within 3 to 8 um, each of said band passfilters removably interposed in said optical path upon direction of anoperator for filtering the infrared rays entering the housing so as toattenuate certain infrared rays and to pass other infrared rays ofparticular, respective predetermined wavelengths associated with saidrange of temperatures of interest, the transmission wavelength of theintervening media and the absorptive wavelength of the targetsurface(s); and, (g) electronic means adapted to convert said electricaloutput into at least one interpretable output (26, 28, 30, 32, 72)whereby an operator is presented with information sufficient todetermine the temperature(s) of the target surface(s) within anacceptable degree of accuracy. 2) The device claimed in claim 1 whereinsaid optical assembly includes an objective lens (74), a negative lens(76), and focusing lens means (18, 82, 84). 3) The device claimed ineither claim 1 or claim 2 wherein, each of said lenses is made ofgermanium. 4) The device claimed in either claim 1 or claim 2 whereinsaid means for optimizing the spectral band width of said opticalassembly to 3 um to 14 um includes each lens having an anti-reflectioncoating with a spectral band width of 3 um to 14 um. 5) The deviceclaimed in claim 3 wherein said means for optimizing the spectral bandwidth of said optical assembly to 3 um to 14 um includes each lenshaving an anti-reflection coating with a spectral band width of 3 um to14 um. 6) The device claimed in either claim 1 or claim 2 wherein saidmeans (84) for optimizing the spectral band width of said UFPA detectorto 3 um to 14 um includes a spectral transmission window (84) positionedin said optical path between said output and said detecting surface,said spectral transmission window having a spectral band width of 3 umto 14 um. 7) The device claimed in claim 3 wherein said means (84) foroptimizing the spectral band width of said UFPA detector to 3 um to 14um includes a spectral transmission window (84) positioned in saidoptical path between said output and said detecting surface, saidspectral transmission window having a spectral band width of 3 um to 14um. 8) The device claimed in claim 4 wherein said means (84) foroptimizing the spectral band width of said UFPA detector to 3 um to 14um includes a spectral transmission window (84) positioned in saidoptical path between said output and said detecting surface, saidspectral transmission window having a spectral band width of 3 um to 14um. 9) The device claimed in claim 5 wherein said means (84) foroptimizing the spectral band width of said UFPA detector to 3 um to 14um includes a spectral transmission window (84) positioned in saidoptical path between said output and said detecting surface, saidspectral transmission window having a spectral band width of 3 um to 14um. 10) The device claimed in claim 1 wherein the thermal imaging oftarget surface(s) can occur in sunlight when said first infrared bandpass filter is interposed in said optical path. 11) The device claimedin claim 1 wherein the spectral band width of said second band passfilter is 3.8 to 4.0 um. 12) The device claimed in claim 1 wherein thespectral band width of said second band pass filter is 4.8 to 5.2 um.13) The device claimed in claim 1 wherein the spectral band width ofsaid second band pass filter is 6.7 to 6.9 um. 14) A device (10) forthermal imaging of target surface(s) having different temperatureswithin a range of temperatures of interest between a high and lowtemperature of 40° C. to 2000° C., the thermal imaging taking placethrough intervening media having a known transmission wavelength, thetarget surface(s) having a known absorptive wavelength, comprising: (a)a housing (12) including an opening (14) for admitting infrared raysincluding those emanating from said target surface(s), said raysdirected along an optical path within said housing, said optical pathhaving an optical axis (38); (b) an optical assembly (40) positionedwithin said housing and in said optical path, said optical assemblyhaving an input and an output, said infrared rays directed towards andinto said input, through and out of said output of said opticalassembly; said optical assembly including an objective lens (74), anegative lens (76), and focusing lens means (18, 82, 84), each of saidlenses made of germanium and having an anti-reflection coating with aspectral band width of 3 um to 14 um; (c) an un-cooled focal planearray, infrared ray detector (UFPA detector) (48) including a detectingsurface (86), said UFPA detector positioned in said housing and in saidoptical path so as to allow the impingement of the infrared rays passingout of said optical assembly onto said detecting surface, said UFPAdetector further including a spectral transmission window (84)positioned in said optical path between said output and said detectingsurface, said spectral transmission window having a spectral band widthof 3 um to 14 um; said UFPA detector providing an electrical outputproportional to the energy of the infrared rays impinging onto saiddetecting surface; (d) filter means (44) including a first (78) andsecond (80) infrared band pass filter, said first infrared band passfilter having a spectral band width of 8 to 14 um, said second infraredband pass filter having a respective spectral band width within 3 to 8um, each of said band pass filters removably interposed in said opticalpath upon direction of an operator for filtering the infrared raysentering the housing so as to attenuate certain infrared rays and topass other infrared rays of particular, respective predeterminedwavelengths associated with said range of temperatures of interest, thetransmission wavelength of the intervening media and the absorptivewavelength of the target surface(s); and, (e) electronic means adaptedto convert said electrical output into at least one interpretable output(26, 28, 30, 32, 72) whereby an operator is presented with informationsufficient to determine the temperature(s) of the target surface(s)within an acceptable degree of accuracy. 15) The device claimed in claim14 wherein the thermal imaging of target surface(s) can occur insunlight when said first infrared band pass filter is interposed in saidoptical path. 16) The device claimed in claim 14 wherein the spectralband width of said second band pass filter is 3.8 to 4.0 um. 17) Thedevice claimed in claim 14 wherein the spectral band width of saidsecond band pass filter is 4.8 to 5.2 um. 18) The device claimed inclaim 14 wherein the spectral band width of said second band pass filteris 6.7 to 6.9 um.